I had the same reservations about removing the blocks from the rails.
I expected a cascade of ball bearings.
Fear not, they will just slide right off. WD40 will be fine to clean them up. If they have scrapers on them, they should be OK internally. You can degrease the rails and go over them with scotchbrite or something else that'll just buff them up a bit. Pay special attention to the grooves that the ball bearings run in, they are top and underside of the rail (you'll figure it out). You might find the blocks feel a little stiff and baulky going back on to the rails, just ease them back and forth until you've got them all the way on, a bit of WD40 helps too.